Japanese Laid-Open Patent Publication No. 2006-52664 discloses a blow-by gas returning apparatus for an internal combustion engine. This blow-by gas returning apparatus is generally provided with a first ventilation passage that connects a portion of an intake passage that is downstream of a throttle valve to a crank chamber, thereby feeding blow-by gas in the crank chamber into the intake passage, a second ventilation passage that connects a portion of the intake passage that is upstream of the throttle valve to the crank chamber, thereby feeding intake air into the intake passage, and an electronically controlled ventilation valve for regulating the flow rate of blow-by gas passing through the first ventilation passage. A demand value of the flow rate of blow-by gas is set based on an engine operating state during operation of the internal combustion engine, and the opening degree of the ventilation valve is controlled such that the actual flow rate of blow-by gas becomes the demand value.
When diluted fuel evaporates from engine lubricant oil with a high fuel dilution ratio in the crank chamber, a large amount of fuel in the crank chamber is fed into the intake air passage together with blow-by gas, and therefore, the actual air-fuel ratio is excessively enriched with respect to the target air-fuel ratio. Thus, it is considered that when the fuel dilution ratio of the engine lubricant oil is high, the ventilation valve may be closed to stop the feed of blow-by gas into the intake air passage. However, since the crank chamber is not ventilated, this is not an effective method.
In the blow-by gas returning apparatus disclosed in the above publication, the actual injection time is fixed to the minimum injection time when a required injection time of an injector is below the minimum injection time, and the ventilation valve is controlled such that the actual air-fuel ratio approaches the target air-fuel ratio, whereby the air-fuel ratio is inhibited from remaining in the state of being excessively enriched. However, even when the air-fuel ratio is excessively enriched until the required injection time falls below the minimum injection time, the ventilation valve is not controlled.